package org.apache.lucene.util;

import java.io.IOException;
import java.io.Reader;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * {@link CharacterUtils} provides a unified interface to Character-related
 * operations to implement backwards compatible character operations based on a
 * {@link Version} instance.
 * 
 * @lucene.internal
 */
public abstract class CharacterUtils {
    private static final Java4CharacterUtils JAVA_4 = new Java4CharacterUtils();
    private static final Java5CharacterUtils JAVA_5 = new Java5CharacterUtils();

    /**
     * Returns a {@link CharacterUtils} implementation according to the given
     * {@link Version} instance.
     * 
     * @param matchVersion
     *          a version instance
     * @return a {@link CharacterUtils} implementation according to the given
     *         {@link Version} instance.
     */
    public static CharacterUtils getInstance(final Version matchVersion) {
        return matchVersion.onOrAfter(Version.LUCENE_31) ? JAVA_5 : JAVA_4;
    }

    /**
     * Returns the code point at the given index of the char array.
     * Depending on the {@link Version} passed to
     * {@link CharacterUtils#getInstance(Version)} this method mimics the behavior
     * of {@link Character#codePointAt(char[], int)} as it would have been
     * available on a Java 1.4 JVM or on a later virtual machine version.
     * 
     * @param chars
     *          a character array
     * @param offset
     *          the offset to the char values in the chars array to be converted
     * 
     * @return the Unicode code point at the given index
     * @throws NullPointerException
     *           - if the array is null.
     * @throws IndexOutOfBoundsException
     *           - if the value offset is negative or not less than the length of
     *           the char array.
     */
    public abstract int codePointAt(final char[] chars, final int offset);

    /**
     * Returns the code point at the given index of the {@link CharSequence}.
     * Depending on the {@link Version} passed to
     * {@link CharacterUtils#getInstance(Version)} this method mimics the behavior
     * of {@link Character#codePointAt(char[], int)} as it would have been
     * available on a Java 1.4 JVM or on a later virtual machine version.
     * 
     * @param seq
     *          a character sequence
     * @param offset
     *          the offset to the char values in the chars array to be converted
     * 
     * @return the Unicode code point at the given index
     * @throws NullPointerException
     *           - if the sequence is null.
     * @throws IndexOutOfBoundsException
     *           - if the value offset is negative or not less than the length of
     *           the character sequence.
     */
    public abstract int codePointAt(final CharSequence seq, final int offset);

    /**
     * Returns the code point at the given index of the char array where only elements
     * with index less than the limit are used.
     * Depending on the {@link Version} passed to
     * {@link CharacterUtils#getInstance(Version)} this method mimics the behavior
     * of {@link Character#codePointAt(char[], int)} as it would have been
     * available on a Java 1.4 JVM or on a later virtual machine version.
     * 
     * @param chars
     *          a character array
     * @param offset
     *          the offset to the char values in the chars array to be converted
     * @param limit the index afer the last element that should be used to calculate
     *        codepoint.  
     * 
     * @return the Unicode code point at the given index
     * @throws NullPointerException
     *           - if the array is null.
     * @throws IndexOutOfBoundsException
     *           - if the value offset is negative or not less than the length of
     *           the char array.
     */
    public abstract int codePointAt(final char[] chars, final int offset, final int limit);

    /**
     * Creates a new {@link CharacterBuffer} and allocates a <code>char[]</code>
     * of the given bufferSize.
     * 
     * @param bufferSize
     *          the internal char buffer size, must be <code>&gt;= 2</code>
     * @return a new {@link CharacterBuffer} instance.
     */
    public static CharacterBuffer newCharacterBuffer(final int bufferSize) {
        if (bufferSize < 2)
            throw new IllegalArgumentException("buffersize must be >= 2");
        return new CharacterBuffer(new char[bufferSize], 0, 0);
    }

    /**
     * Fills the {@link CharacterBuffer} with characters read from the given
     * reader {@link Reader}. This method tries to read as many characters into
     * the {@link CharacterBuffer} as possible, each call to fill will start
     * filling the buffer from offset <code>0</code> up to the length of the size
     * of the internal character array.
     * <p>
     * Depending on the {@link Version} passed to
     * {@link CharacterUtils#getInstance(Version)} this method implements
     * supplementary character awareness when filling the given buffer. For all
     * {@link Version} &gt; 3.0 {@link #fill(CharacterBuffer, Reader)} guarantees
     * that the given {@link CharacterBuffer} will never contain a high surrogate
     * character as the last element in the buffer unless it is the last available
     * character in the reader. In other words, high and low surrogate pairs will
     * always be preserved across buffer boarders.
     * </p>
     * 
     * @param buffer
     *          the buffer to fill.
     * @param reader
     *          the reader to read characters from.
     * @return <code>true</code> if and only if no more characters are available
     *         in the reader, otherwise <code>false</code>.
     * @throws IOException
     *           if the reader throws an {@link IOException}.
     */
    public abstract boolean fill(CharacterBuffer buffer, Reader reader) throws IOException;

    private static final class Java5CharacterUtils extends CharacterUtils {
        Java5CharacterUtils() {
        }

        @Override
        public final int codePointAt(final char[] chars, final int offset) {
            return Character.codePointAt(chars, offset);
        }

        @Override
        public int codePointAt(final CharSequence seq, final int offset) {
            return Character.codePointAt(seq, offset);
        }

        @Override
        public int codePointAt(final char[] chars, final int offset, final int limit) {
            return Character.codePointAt(chars, offset, limit);
        }

        @Override
        public boolean fill(final CharacterBuffer buffer, final Reader reader) throws IOException {
            final char[] charBuffer = buffer.buffer;
            buffer.offset = 0;
            charBuffer[0] = buffer.lastTrailingHighSurrogate;
            final int offset = buffer.lastTrailingHighSurrogate == 0 ? 0 : 1;
            buffer.lastTrailingHighSurrogate = 0;
            final int read = reader.read(charBuffer, offset, charBuffer.length - offset);
            if (read == -1) {
                buffer.length = offset;
                return offset != 0;
            }
            buffer.length = read + offset;
            // special case if the read returns 0 and the lastTrailingHighSurrogate was set
            if (buffer.length > 1 && Character.isHighSurrogate(charBuffer[buffer.length - 1])) {
                buffer.lastTrailingHighSurrogate = charBuffer[--buffer.length];
            }
            return true;
        }
    }

    private static final class Java4CharacterUtils extends CharacterUtils {
        Java4CharacterUtils() {
        }

        @Override
        public final int codePointAt(final char[] chars, final int offset) {
            return chars[offset];
        }

        @Override
        public int codePointAt(final CharSequence seq, final int offset) {
            return seq.charAt(offset);
        }

        @Override
        public int codePointAt(final char[] chars, final int offset, final int limit) {
            if (offset >= limit)
                throw new IndexOutOfBoundsException("offset must be less than limit");
            return chars[offset];
        }

        @Override
        public boolean fill(final CharacterBuffer buffer, final Reader reader) throws IOException {
            buffer.offset = 0;
            final int read = reader.read(buffer.buffer);
            if (read == -1)
                return false;
            buffer.length = read;
            return true;
        }

    }

    /**
     * A simple IO buffer to use with
     * {@link CharacterUtils#fill(CharacterBuffer, Reader)}.
     */
    public static final class CharacterBuffer {

        private final char[] buffer;
        private int offset;
        private int length;
        private char lastTrailingHighSurrogate = 0;

        CharacterBuffer(char[] buffer, int offset, int length) {
            this.buffer = buffer;
            this.offset = offset;
            this.length = length;
        }

        /**
         * Returns the internal buffer
         * 
         * @return the buffer
         */
        public char[] getBuffer() {
            return buffer;
        }

        /**
         * Returns the data offset in the internal buffer.
         * 
         * @return the offset
         */
        public int getOffset() {
            return offset;
        }

        /**
         * Return the length of the data in the internal buffer starting at
         * {@link #getOffset()}
         * 
         * @return the length
         */
        public int getLength() {
            return length;
        }

        /**
         * Resets the CharacterBuffer. All internals are reset to its default
         * values.
         */
        public void reset() {
            offset = 0;
            length = 0;
            lastTrailingHighSurrogate = 0;
        }
    }

}
